This invention relates to a digital convergence correction device used for the correction of distortion and correction of convergence for a cathode ray tube screen of a television receiver or display unit.
The prevalence of VTRs and video disks is followed by intense demands of powerful pictures, and the television is now in a phase of offering advanced large screens and home theaters. The result is the wide use of a so-called projection-type televison in which a picture reproduced on the fluorescent screen of a Braun tube is projected through enlargement onto a screen by means of a projection optical system including lenses and mirrors thereby to make a large frame of picture. The projection-type television is superior in chromatic aberration, spherical aberration, comatic aberration, astigmatism, and image curvature, and has improved sharpness (focus) of the image through the use of aspherical surface plastics, aspherical surface projection tube fluorescent screen and short projection optical system.
However, each portion of the screen has a different magnification, creating a so-called distortion in which the figure of an object and its image are not similar. For the distortion, two parameters, i.e., the absolute value and high-order distortion need to be considered. Generally, in the optical design, the above-mentioned aberration and distortion are contradictory, and a compromised point between the two factors is chosen for the design value.
Accordingly, an endless pursuit of the focus performance results in an increased distortion, especially an increased high-order distortion. In addition, the projection-type television operates to form an image on the screen by collecting three-color light beams from three projectors of R, G and B in different directions, and therefore the convergence performance becomes more complicated due to increased distortion.
As a method of correcting the distortion, there is described a technique of analog convergence correction in publication entitled "Color Television Textbook", pp. 262-265, published by Japanese Broadcasting Corporation, for example. This method combines a saw tooth wave and parabolic wave in synchronism with the horizontal sync signal and vertical sync signal to form a correction current and feeds the current to a convergence yoke (will be termed simply CY) thereby to implement the distortion correction and convergence correction.
However, this method is limited in combinations of waveforms, and therefore it can not completely correct the distortion of frame and the color shaft.
On this account, it is indispensable to have a digital convergence correction device which stores arbitrary waveforms which match complex distortions in a memory and converts the waveform data into analog waveforms to drive the convergence yokes.
The conventional digital convergence correction devices accomplish the correction through the provision of a D/A converter for each of the convergence yoke and convergence coil, as described in JP-A-50-68012 and JP-A-61-12191, and accordingly television receivers which use many convergence yokes and convergence coils need as many D/A converters as the number of reception channels. In this method, the screen of the cathode ray tube is partitioned in the horizontal and vertical directions, with the intersections of border lines being set as the convergence adjustment points, predetermined convergence correction values at these adjustment points are stored as digital correction data in a digital memory, the digital correction data is read out of the memory and converted into analog signals with digital-to-analog (D/A) converters, and the signals are fed to the convergence yokes thereby to implement the convergence correction.
In any of cases of a single image reception tube (cathode ray tube) and three image reception tubes for red, green and blue as in a projection-type television receiver, many convergence yokes are often provided for the tube, and in such cases digital correction data for each convergence yoke is read out of the memory and, after conversion into the analog signal with the D/A converter, it is fed to the convergence yoke thereby to implement the convergence correction.
However, this design necessitates D/A converters equal in number to the convergence yokes to e driven, leaving a problem of high cost.
In order to overcome this problem, there is proposed a digital convergence correction device, in which digital correction data for all convergence yokes are read out of a digital memory concurrently, the data are rendered the time division multiplexing with a multiplexer to form serial digital correction data, and it is converted into analog signals with a D/A converter and fed to the respective convergence yokes. This conventional digital convergence correction device will be explained in the following.
FIG. 8 is a block diagram showing the circuit arrangement of the conventional digital convergence correction device which is applied to the three-tube projection-type television receiver. In the figure, indicated by 1 is an input terminal for the horizontal blanking pulse (H. BLK) used for raster scanning, and 2 is an input terminal for the vertical blanking pulse (V. BLK). 3 is a sync pulse generation circuit which produces a clock pulse (not shown) synchronized to the blanking pulses, a horizontal sync pulse 4 and a vertical sync pulse 5, 6 is a horizontal address generation circuit, and 7 is a vertical address generation circuit for reading out a memory 8.
Indicated by 11 and 12 are correction value data which will become the signals for driving a horizontal convergence coil 91 and vertical convergence coil 92 wound on a convergence yoke 101 provided for a red (R) projection tube 111, 13 and 14 are correction value data which will become the signals for driving a horizontal convergence coil 93 and vertical convergence coil 94 provided for a green (G) projection tube 112, and 15 and 16 are correction value data which will become the signals for driving a horizontal convergence coil 95 and vertical convergence coil 96 provided for a blue (B) projection tube 113.
Indicated by 9 is a multiplexer which implements the time division multiplexing for the above-mentioned six correction data in response to the pulses produced by a six-channel time division pulse generation circuit 18, and 17 is multiplexed data resulting from the time division multiplexing for the correction value data 11-16. 10 is a D/A converter which converts the multiplexed digital data into an analog signal 19, 21-26 are sample-holding circuits which restore the individual signal waveforms from the multiplexed analog signal and hold the resulting signals, 30 in a sample-holding pulse generation circuit which generates pulses 31-36 in different phases for driving the sample-holding circuits, 71-76 are low-pass filters (LFSs) used for interpolation, and 81-86 are convergence yoke amplifiers for driving the convergence yokes.
FIG. 9 is a timing chart showing the timing relation among the major signals of the circuit shown in FIG. 8. It is assumed that correction data strings 11, 12 and 16 (13-15 are skipped for the simplicity of explanation) read out of the memory 8 by the address signals are data (RH1, GH1, BV1) in the timing relation shown by a, b and c in FIG. 9. The correction value data strings 11, 12 and 16 are generally data strings of RHi, RBi and BVi and (i=1, 1, . . . , n), respectively.
The multiplexer 9 in FIG. 8 selects the correction data strings 11-16 periodically in response to pulse generated by the six-channel time division pulse generation circuit 18 to form a multiplex data string 17 shown by d in FIG. 9 through the time slicing for the six data strings 11-16. The multiplex data string 17 shown by d in FIG. 9 includes a total of eight data, i.e., RH1, GH1, BH1, RV1, GV1 and BV1 plus two pieces of blank data shown by hatching, instead of the former six data which are required inherently, with the reason for eight being a number 2 to the power 3 for the technical convenience in time division multiplexing.
The multiplex data string 17 is fed to the D/A converter 10, which then produces an analog signal shown by e in FIG. 9. The analog signal is fed to the sample-holding circuits 21, 22 and 26 which operate in response to sample-holding pulses 31, 32 and 36 in different phases shown by f, h and j in FIG. 9. Each sample-holding circuit operates to sample the input signal when the sample-holding pulse is high, and it operates to hold the signal when the pulse is low. Consequently, the sample-holding circuits 21, 22 and 26 extract analog signals 41, 42 and 46 shown by g, i and k in FIG. 9.
The analog signals are fed through the low-pass filters (LPFs) 71, 72 and 76 and the convergence yoke amplifiers 81, 82 and 86 so that the convergence yokes 91, 92 and 96 are driven for the convergence correction.
The foregoing conventional digital convergence correction device is advantageous economically since it needs only one D/A converter regardless of the number of convergence yokes to be driven, but on the other hand it suffers from the difficult maneuver of convergence correction which is based on the analog signals of different phases for each channel. The reason for the difficult convergence adjustment will be explained in detail in connection with FIG. 10 showing the screen of a television receiver during the convergence adjustment.
In FIG. 10, indicated by 801 is the frame of the screen. Vertical lines and horizontal lines inside the screen frame 801 are a cross hatch used for the reference of convergence adjustment, and each intersection 802 of a vertical and a horizontal lines is an adjustment point at which the convergence adjustment takes place.
FIG. 11 is a diagram showing the relation between the convergence correction waveforms and the screen positions based on the conventional digital convergence correction device shown in FIG. 8. In the figure, shown by 803 is the partial enlargement of the cross hatch of FIG. 10, indicating the positions on the screen. Indicated by 811, 812 and 816 are the outputs of the sample-holding circuits 21, 22 and 26 in FIG. 8, i.e., the correction waveforms for the convergence yokes 91, 92 and 96 in implementing the convergence correction based on the output signals shown by g, i and k in FIG. 9. The waveforms 811, 812 and 816 have their horizontal axes corresponding to the screen position 803, with the vertical axes representing the level of the convergence yoke correction waveforms.
The waveforms of FIG. 11 reveal that the convergence yoke correction waveforms 811-816 have their peak positions different among the convergence yokes. For the accurate convergence adjustment, it is necessary to assure the state of convergence at the peak position of the convergence yoke correction waveform during the adjustment. It obliges the verification of the convergence yoke correction waveform during the adjustment. It obliges the verification of the convergence state at different screen positions depending on the color and mode of horizontal or vertical correction, resulting in a difficult maneuver of convergence adjustment.